Quality of precision time sources

ABSTRACT

Systems and methods for detecting the failure of a precision time source using an independent time source are disclosed. Additionally, detecting the failure of a GNSS based precision time source based on a calculated location of a GNSS receiver is disclosed. Moreover, the system may be further configured to distribute a time derived from the precision time source as a precision time reference to time dependent devices. In the event of a failure of the precision time source, the system may be configured to distribute a time derived from a second precision time source as the precision time signal during a holdover period.

RELATED APPLICATION

The present application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/698,583 filed Sep. 8, 2012 and titled “Quality of Precision Time Sources,” which application is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to detecting the failure of a precision time source using an independent time source. Particularly, this disclosure relates to detecting the failure of a precision time source in an electric power transmission or distribution system.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the disclosure are described, including various embodiments of the disclosure with reference to the figures, in which:

FIG. 1 is a one-line diagram of an electric power delivery system.

FIG. 2 illustrates a time distribution system including communications IEDs configured to distribute a precision time reference to various IEDs.

FIG. 3 illustrates an embodiment of a time distribution device configured to receive, distribute, and/or determine a precision time reference.

FIG. 4 illustrates one embodiment for determining whether a primary or best available time source has failed.

FIG. 5 illustrates another embodiment for determining whether a primary or best available time source has failed.

FIG. 6 illustrates one embodiment for determining whether a primary or best available time source has failed based on GNSS location.

In the following description, numerous specific details are provided for a thorough understanding of the various embodiments disclosed herein. However, those skilled in the art will recognize that the systems and methods disclosed herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In addition, in some cases, well-known structures, materials, or operations may not be shown or described in detail in order to avoid obscuring aspects of the disclosure. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more alternative embodiments.

DETAILED DESCRIPTION

Electric power transmission and distribution systems may utilize precision time information to perform various monitoring, protection, and communication tasks. In connection with certain applications, intelligent electronic devices (IEDs) and network communication devices may utilize time information accurate beyond the millisecond range. IEDs within a power system may be configured to perform metering, control, and protection functions that require a certain level of precision between one or more IEDs. For example, IEDs may be configured to calculate and communicate time-synchronized phasors (synchrophasors), which may require that the IEDs and network devices be synchronized to within nanoseconds of one other. Many protection, metering, control, and automation algorithms used in power systems may benefit from or require receipt of precision time information.

Various systems may be used for distribution of precision time information. According to various embodiments disclosed herein, a power system may include components connected using a synchronized optical network (SONET). In such embodiments, precision time information may be distributed using a synchronous transport protocol and synchronous transport modules (STMs). According to one embodiment, a precision time reference can be transmitted within a frame of a SONET transmission. In another embodiment, a precision time reference may be incorporated into a header or an overhead portion of a SONET STM frame. Similarly, the power system may include components connected using Synchronous Digital Hierarchy (SDH) protocol. Although several embodiments herein are described in terms of SONET, it should be recognized that the SDH protocol may be used in place of SONET unless otherwise specified.

IEDs, network devices, and other devices in a power system may include local oscillators or other time sources and may generate a local time signal. In some circumstances, however, external time signals, provided by a time distribution device, may be more precise and may therefore be preferred over local time signals. A power system may include a data communications network that transmits a precision time reference from the time distribution device to time dependent devices connected to the data communications network. In some embodiments, the communications network may include one or more local area networks (LANs) and one or more wide area networks (WANs). In a system with multiple LANs, multiple time distribution devices (one or more for each LAN) may be connected to the data communications network and each time distribution device can provide a precision time reference to other time distribution devices across the WAN. In each time distribution device, the precision time reference may be received or derived from an external precision time signal.

According to various embodiments, each time distribution device receives multiple precision time signals from various time sources and is configured to provide the best available precision time signal as the precision time reference. The precision time signals may be received using an Inter-Range Instrumentation Group (IRIG) protocol, a global navigation satellite system (GNSS), such as, for example, global positioning system (GPS), GLONASS, or the like, a radio broadcast such as a National Institute of Science and Technology (NIST) broadcast (e.g., radio stations WWV, WWVB, and WWVH), the IEEE 1588 protocol, a network time protocol (NTP) codified in RFC 1305, a simple network time protocol (SNTP) in RFC 2030, and/or another time transmission protocol or system.

While, the above listed precision time signals may provide accurate time to a time distribution device, they vary in quality. For example, the precision of NTP and SNTP is limited to the millisecond range, thus making it inappropriate for sub-millisecond time distribution applications. Further, both protocols lack security and are susceptible to malicious network attacks. The IEEE 1588 standard includes hardware-assisted timestamps, which allows for time accuracy in the nanosecond range. Such precision may be sufficient for more demanding applications (e.g., the sampling of the sinusoidal currents and voltages on power lines to calculate “synchrophasors”). It is well suited for time distribution at the communication network periphery, or among individual devices within the network. GNSS time signals provide a very accurate and robust time measurement, however GNSS signals are susceptible to spoofing. Therefore, it would be beneficial to provide a system and method for detecting failure in any of the received precision time signals such that the best available precision time reference can be provided to time dependent devices.

In certain embodiments, when the time distribution device determines that the connection to the best available time source has failed, a new best available time source may be selected from the remaining available time sources. In addition to relying on a precision time reference from the time distribution device, when available, the various time dependent devices may be configured to enter a holdover period when the precision time reference is unavailable. In some embodiments, a device may be configured to monitor the drift of a local time source with respect to the precision time reference and to retain information regarding the drift. During the holdover period, an IED or network device may rely on a local time signal.

Reference throughout this specification to “one embodiment” or “an embodiment” indicates that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. In particular, an “embodiment” may be a system, an article of manufacture (such as a computer readable storage medium), a method, and a product of a process.

The phrases “connected to,” “networked,” and “in communication with” refer to any form of interaction between two or more entities, including mechanical, electrical, magnetic, and electromagnetic interaction. Two components may be connected to each other even though they are not in direct physical contact with each other and even though there may be intermediary devices between the two components.

Some of the infrastructure that can be used with embodiments disclosed herein is already available, such as: general-purpose computers, computer programming tools and techniques, digital storage media, and optical networks. A computer may include a processor such as a microprocessor, microcontroller, logic circuitry, or the like. The processor may include a special purpose processing device such as an ASIC, PAL, PLA, PLD, Field Programmable Gate Array, or other customized or programmable device. The computer may also include a computer readable storage device such as non-volatile memory, static RAM, dynamic RAM, ROM, CD-ROM, disk, tape, magnetic, optical, flash memory, or other computer readable storage medium.

As used herein, the term IED may refer to any microprocessor-based device that monitors, controls, automates, and/or protects monitored equipment within the system. Such devices may include, for example, remote terminal units, differential relays, distance relays, directional relays, feeder relays, overcurrent relays, voltage regulator controls, voltage relays, breaker failure relays, generator relays, motor relays, automation controllers, bay controllers, meters, recloser controls, communications processors, computing platforms, programmable logic controllers (PLCs), programmable automation controllers, input and output modules, and the like. IEDs may be connected to a network, and communication on the network may be facilitated by networking devices including, but not limited to, multiplexers, routers, hubs, gateways, firewalls, and switches. Furthermore, networking and communication devices may be incorporated in an IED or be in communication with an IED. The term IED may be used interchangeably to describe an individual IED or a system comprising multiple IEDs.

IEDs, network devices, and time distribution devices may be physically distinct devices, may be composite devices, or may be configured in a variety of ways to perform overlapping functions. IEDs, network devices, and time distribution devices may comprise multi-function hardware (e.g., processors, computer-readable storage media, communications interfaces, etc.) that can be utilized in order to perform a variety of tasks, including tasks typically associated with an IED, network device, and/or time distribution device. For example, a network device, such as a multiplexer, may also be configured to issue control instructions to a piece of monitored equipment. In another example, an IED may be configured to function as a firewall. The IED may use a network interface, a processor, and appropriate software instructions stored in a computer-readable storage medium in order to simultaneously function as a firewall and as an IED. In another example, an IED may include the necessary hardware and software instructions to function as a time distribution device for other IEDs in a LAN or WAN. In order to simplify the discussion, several embodiments disclosed herein are illustrated in connection with time distribution devices; however, one of skill in the art will recognize that the teachings of the present disclosure, including those teachings illustrated only in connection with time distribution devices, are also applicable to IEDs and network devices.

Aspects of certain embodiments described herein may be implemented as software modules or components. As used herein, a software module or component may include any type of computer instruction or computer executable code located within a computer readable storage medium. A software module may, for instance, comprise one or more physical or logical blocks of computer instructions, which may be organized as a routine, program, object, component, data structure, etc., that performs one or more tasks or implements particular abstract data types.

In certain embodiments, a particular software module may comprise disparate instructions stored in different locations of a computer readable storage medium, which together implement the described functionality of the module. Indeed, a module may comprise a single instruction or many instructions, and may be distributed over several different code segments, among different programs, and across several computer readable storage media. Some embodiments may be practiced in a distributed computing environment where tasks are performed by a remote processing device linked through a communications network. In a distributed computing environment, software modules may be located in local and/or remote computer readable storage media. In addition, data being tied or rendered together in a database record may be resident in the same computer readable storage medium, or across several computer readable storage media, and may be linked together in fields of a record in a database across a network.

The software modules described herein tangibly embody a program, functions, and/or instructions that are executable by computer(s) to perform tasks as described herein. Suitable software, as applicable, may be readily provided by those of skill in the pertinent art(s) using the teachings presented herein and programming languages and tools, such as XML, Java, Pascal, C++, C, database languages, APIs, SDKs, assembly, firmware, microcode, and/or other languages and tools.

A precision time reference refers to a time signal or time source relied on by a plurality of devices and distributed by a time distribution device, and which is presumed to be more precise than a local time source. The determination of accuracy may be made based upon a variety of factors. A precision time reference may allow for specific moments in time to be described and temporally compared to one another.

A time source is any device that is capable of tracking the passage of time. A variety of types of time sources are contemplated, including a voltage-controlled temperature compensated crystal oscillator (VCTCXO), a phase locked loop oscillator, a time locked loop oscillator, a rubidium oscillator, a cesium oscillator, a trained oscillator, a microelectromechanical device (MEM), and/or other device capable of tracking the passage of time.

A time signal is a representation of the time indicated by a time source. A time signal may be embodied as any form of communication for communicating time information. A wide variety of types of time signals are contemplated, such as those listed above. Time source and time signal may be used interchangeably herein.

Failure of a precision time source and/or precision time signal, as used herein, includes spoofing and/or jamming the signal, mechanical or software failures, system wide outages, etc.

Furthermore, the described features, operations, or characteristics may be combined in any suitable manner in one or more embodiments. It will also be readily understood that the order of the steps or actions of the methods described in connection with the embodiments disclosed herein may be changed, as would be apparent to those skilled in the art. Thus, any order in the drawings or detailed description is for illustrative purposes only and is not meant to imply a required order, unless specified to require an order.

FIG. 1 illustrates a one-line diagram of an electric power delivery system 10. The delivery system 10 includes intelligent electronic devices (IEDs) 102, 104, and 106 utilizing a precision time reference to monitor, protect, and/or control system components. The electric power transmission and delivery system 10 illustrated in FIG. 1 includes three geographically separated substations 16, 22, and 35. Substations 16 and 35 include generators 12 a, 12 b, and 12 c. The generators 12 a, 12 b, and 12 c generate electric power at a relatively low voltage, such as 12 kV. The substations include step-up transformers 14 a, 14 b, and 14 c to step up the voltage to a level appropriate for transmission. The substations include various breakers 18 and buses 19, 23, and 25 for proper transmission and distribution of the electric power. The electric power may be transmitted over long distances using various transmission lines 20 a, 20 b, and 20 c.

Substations 22 and 35 include step-down transformers 24 a, 24 b, 24 c, and 24 d for stepping down the electric power to a level suitable for distribution to various loads 30, 32, and 34 using distribution lines 26, 28, and 29.

IEDs 102, 104, and 106 are illustrated in substations 16, 22, and 35 configured to protect, control, meter and/or automate certain power system equipment or devices. According to several embodiments, numerous IEDs are used in each substation; however, for clarity only a single IED at each substation is illustrated. IEDs 102, 104, and 106 may be configured to perform various time dependent tasks including, but not limited to, monitoring and/or protecting a transmission line, distribution line, and/or a generator. Other IEDs included in a substation may be configured as bus protection relays, distance relays, communications processors, automation controllers, transformer protection relays, and the like. As each IED or group of IEDs may be configured to communicate on a local area network (LAN) or wide area network (WAN), each IED or group of IEDs may be considered a node in a communications network.

As discussed above, an IED may be configured to calculate and communicate synchrophasors with other IEDs. To accurately compare synchrophasors obtained by geographically separate IEDs, each IED may need to be synchronized with a precision time reference with accuracy greater than a millisecond to allow for time-aligned comparisons. According to various embodiments, time synchronization, accurate to the microsecond or nanosecond range, may allow IEDs to perform accurate comparisons of synchrophasors.

FIG. 2 illustrates system 200 configured to be a highly reliable, redundant, and distributed system of time distribution devices 204, 206, and 208 capable of providing a precision time reference to various time dependent IEDs 212, 214, and 216. Each time distribution device 204, 206, and 208 may be configured to receive and communicate time signals through multiple protocols and methods. While the system 200 is described as being capable of performing numerous functions and methods, it should be understood that various systems are possible that may have additional or fewer capabilities. Specifically, a system 200 may function as desired using only one protocol, or having fewer external or local time signal inputs.

As illustrated in FIG. 2, three time distribution devices 204, 206, and 208 have WAN capabilities and are communicatively connected to a WAN 218, which may comprise one or more physical connections and protocols. Each time distribution device 204, 206, and 208 may also be connected to one or more IEDs within a local network. For example, time distribution device 204 is connected to IED 212, time distribution device 206 is connected to IEDs 214, and time distribution device 208 is connected to IEDs 216. A time distribution device may be located at, for example, a power generation facility, a hub, a substation, a load center, or other location where one or more IEDs are found. In various embodiments, an IED may include a WAN port, and such an IED may be directly connected to WAN 218. IEDs may be connected via WAN 218 or LANs 210. Time distribution devices 204, 206, and 208 may establish and maintain a precision time reference among various system components. Each time distribution device 204, 206, and 208 may be configured to communicate time protocols, such as IEEE 1588.

Each time distribution device 204, 206, and 208 is configured to receive time signals from a variety of time sources. For example, as illustrated, time distribution device 204 includes an antenna 220 and is configured to receive a GNSS signal from a GNSS repeater or satellite 202. Time distribution device 204 is also configured to receive a second time signal 221 from an external time source 201. The external time source may comprise one or more VCTCXOs, phase locked loop oscillators, time locked loop oscillators, rubidium oscillators, cesium oscillators, NIST broadcasts (e.g., WWV and WWVB), and/or other devices capable of generating precise time signals. In the illustrated embodiment, time distribution device 208 includes an antenna 220 configured to receive a GNSS signal from the GNSS repeater or satellite 202. As illustrated, time distribution device 206 does not directly receive an external time signal, however, according to alternative embodiments, any number and variety of external time signals may be available to any of the time distribution devices.

According to one embodiment, WAN 218 comprises a SONET configured to embed a precision time reference in a header or overhead portion of a SONET frame during transmission. Alternatively, a precision time reference may be conveyed using any number of time communications methods including IRIG protocols, NTP, SNTP, synchronous transport protocols (STP), and/or IEEE 1588 protocols. According to various embodiments, including transmission via SONET, a precision time reference may be separated and protected from the rest of the WAN network traffic, thus creating a secure time distribution infrastructure. Protocols used for inter IED time synchronization may be proprietary, or based on a standard, such as IEEE 1588 Precision Time Protocol (PTP).

According to various embodiments, time distribution devices 204, 206, and 208 are configured to perform at least one of the methods of detecting failure of a time source described herein. System 200 may utilize a single method or combination of methods, as described herein.

It is of note that even the most precise time signals may exhibit small discrepancies. For example, depending on the length and routing of the GNSS antenna cable, various clocks may exhibit microsecond level time offsets. Some of these offsets may be compensated by the user entering compensation settings, or may need to be estimated by the time synchronization network. Estimation may be performed during long periods of “quiet” operation (i.e., periods with no faults), with the individual source results stored locally in a nonvolatile storage register.

FIG. 3 illustrates a time distribution device 304, according to one embodiment. A time distribution device 304 may include more or less functionality than the illustration. For example, time distribution device 304 may include an interface for monitoring equipment in an electric power delivery system in certain embodiments. Accordingly, in various embodiments time distribution device 304 may be implemented either as an IED or as a network device. As illustrated, time distribution device 304 includes a local time source 302 that provides a local time signal and a time quality module 305 for establishing a precision time reference. Time distribution device 304 further includes a pair of line ports 312 and 314 for communications with a WAN or LAN. Time information may be shared over a network and may also be fed into the time quality module 305. Further, time distribution device 304 includes a GNSS receiver 310 for receiving a precision time signal, such as time from a GNSS via a GNSS antenna 320. Time distribution device 304 also includes a WWVB receiver 330 for receiving an NIST broadcast, which can be used as a precision time signal, via an external antenna 340. The received precision time signal from either source is communicated to the time quality module 305 for use in determining and distributing the precision time reference.

Another time source that may be fed to the time quality module 305 includes an external time source 306 that may conform to a time distribution protocol, such as IRIG. The external time source 306 may communicate with another time port such as an IRIG input 308.

The various time information from the WAN (from line ports 312 and/or 314), GNSS receiver 310, WWVB receiver 330, and IRIG input 308 are input into the time quality module 305. In one embodiment, the inputs may be fed into a multiplexer (not shown) prior to being input into the time quality module 305. The time quality module 305 functions to determine a precision time reference for use by the various devices connected to the time distribution device 304. The precision time reference is then communicated from the time quality module 305 to the various devices 322 using IRIG protocol (via the IRIG-B output 316) or to various devices 325 using another protocol 313 such as IEEE 1588 using Ethernet Drop Ports 318. The Ethernet Drop Ports 318 may also include network communications to the various devices connected to time distribution device 304. Time distribution device 304 may further include connections to SONETs and transmit the precision time reference in a header or overhead portion of SONET frames.

Time distribution device 304 may also comprise a time signal adjustment subsystem 324. Time signal adjustment subsystem 324 may be configured to track drift rates associated with various external time sources with respect to local time source 302. Time signal adjustment subsystem 324 may also communicate time signals according to a variety of protocols. Such protocols may include inter-Range Instrumentation Group protocols, IEEE 1588, Network Time Protocol, Simple Network Time Protocol, synchronous transport protocol, and the like. In various embodiments, time signal adjustment subsystem 324 may be implemented using a processor in communication with a computer-readable storage medium containing machine executable instructions. In other embodiments, time signal adjustment subsystem 324 may be embodied as hardware, such as an application specific integrated circuit or a combination of hardware and software.

According to various embodiments, the time quality module 305 determines whether a primary or “best available” time source is reliable, i.e., has not failed, and distributes the time signal from the best available time source as the precision time reference to time dependent devices in the system. If the best available time source has failed, the time quality module 305 provides an error alert to a user, and in some embodiments, enters a holdover period where an alternative time signal is used for the precision time reference. These techniques allow for the best available time source to be used as precision time reference provided to time dependent devices in a robust manner such that there is a high likelihood that the precision time reference is accurate. Moreover, in certain embodiments, relying on a secondary time source provided to the time quality module 305 as the precision time reference during a holdover period when the primary time reference has failed may provide more accurate time information than the holdover situation described above where a local oscillator in each time dependent device is used during the holdover.

In some embodiments, after a period of time using the secondary time source, a primary time source may become available again. The time quality module 305 may determine whether the primary time source is reliable. If the primary time source is reliable, the time distribution device 304 may begin using the primary time source for the precision time reference. If however, the primary time source is determined to be unreliable, the time distribution device 304 may continue using the secondary time source for the precision time reference and provide an error alert to a user indicating the availability and unreliability of the primary time source.

FIG. 4 illustrates one embodiment for determining whether a primary or best available time source has failed. While the time signals in the example of FIG. 4 are described as specific signals, other signals may be used with similar results. At 402 the time distribution device receives a first time signal from a first time source, or best available time source, and provides the time signal to the time quality module. In one embodiment, the first time source is a time signal received from a GNSS system. GNSS time has the advantages of relying on extremely accurate methods for providing the time signal to GNSS receivers, being readily available worldwide (particularly in remote locations) 24 hours per day, and is expected to be stable for many decades to come. GNSS receivers can keep an internal time, based on the GNSS signal that is accurate to better than nanoseconds and the time output at the 1 PPS dedicated time port is typically better than 1 microsecond.

At 404 the time distribution device receives a second time signal from a second time source. In one embodiment, the second time source is a NIST broadcast such as WWVB. While not as accurate as a time reference derived from a GNSS signal, a time reference derived from a WWVB broadcast is still very accurate. While the example of FIG. 4 specifically uses a WWVB broadcast as the second time source, one of skill in the art will recognize that other time sources, such as those described above, can be used in place of the WWVB broadcast.

At 406 the time quality module compares the first time signal to the second time signal. Each of the time signals received by the time quality module have an inherent error bound related to the accuracy of the time signal. In one embodiment, the time quality module compares the time signals with regard to their respective error bounds to determine whether the first time source has failed. For example, given the relatively smaller error bound found in the time derived from a GNSS signal compared to that found in a time derived from a WWVB broadcast, the time based on the GNSS signal should fall within the error bound of the time based on the WWVB broadcast. However, if the GNSS based time signal falls outside of the error bound of the WWVB based time signal, the time quality module detects, at 408, that there is an error with the GNSS based time signal.

If, at 408, the time quality module determines that the first time source has not failed, the time quality module distributes time from the first time signal as the precision time reference at 410. If, at 408, the time quality module determines that the first time source has failed, at 412 the time quality module alerts a user that the best available time source has failed and that the time may not be accurate. In addition to alerting a user of the failure, the time quality module at 414 can optionally distribute time from the second time signal as the precision time reference.

While the example of FIG. 4 is limited to a first and second time signal, the time quality module can continue to compare time signals in order of relative error bounds beyond just a first and second time signal. For example, the WWVB based time may be compared to the time of a local oscillator (taking into account the drift rate of the oscillator) to determine whether the WWVB source has failed, etc.

FIG. 5 illustrates a second embodiment for determining whether a primary or best available time source has failed. While the time signals in the example of FIG. 5 are described as specific signals, other signals may be used with similar results. At 502 the time distribution device receives a first time signal from a first time source, or best available time source, and provides the time signal to the time quality module. In one embodiment, the first time source is a time signal received from a GNSS system.

At 504 the time distribution device uses the first time signal to train an unlocked oscillator to track the time provided in the first time signal. While the oscillator is trained to track the time of the first time source, because the oscillator is unlocked the time provided by the trained oscillator will drift from that of the first time signal. However, the rate of drift is low and the time distribution device maintains the training relationship between the first signal and the oscillator such that the drift is corrected.

At 506 the time quality module compares the first time signal to the trained oscillator (again, taking into account the drift rate associated with the trained oscillator). In one embodiment, a counter tracks the number of oscillations of the oscillator between each PPS received from the first time signal. Because the oscillator is trained to the first time signal, any variation in the oscillation count from PPS to PPS should be low. If there is a large jump in the variation in the oscillation count, the time quality module, at 508, detects a failure of the first time source. The threshold for determining whether the time quality module detects a failure of the time source may depend on the characteristics of the oscillator used. For example, a temperature compensated crystal oscillator (TCXO) may have a draft rate in the parts-per-million range while oven controlled crystal oscillators and cesium based oscillators may have a drift rate in the parts-per-billion. Thus, the threshold for the more accurate oscillator may be higher. If the variation in the oscillation count exceeds the threshold, the time quality module may indicate a failure of the first time source.

In another embodiment, the oscillator may be used to validate time quality measurements transmitted as part of the time source. For example, an IRIG signal includes a Time Quality and Continuous Time Quality indication. The time quality module may use the oscillator to validate the time quality signal received as part of the time source.

If, at 508, the time quality module determines that the first time source has not failed, the time quality module distributes time from the first time signal as the precision time reference at 510. If, at 508, the time quality module determines that the first time source has failed, at 512 the time quality module alerts a user that the best available time source has failed and that the time may not be accurate. In addition to alerting a user of the failure, the time quality module at 514 can optionally distribute time from the trained oscillator, or a second time source, as the precision time reference during a holdover period.

In some embodiments, if a second time source is to be used for the precision time reference, the time quality module may determine whether the second time source is acceptable. For example, the time quality module may determine whether the time provided by the second time source falls within an acceptable range of the current time provided by the time distribution device (e.g., time kept by the internal oscillator). If the time falls within the acceptable range, the second time source may be used to provide the precision time reference.

The example embodiments above provide for a robust system of providing a precision time reference to time dependent devices by comparing several time signals to determine whether the best available time source has failed. FIG. 6 illustrates one embodiment for determining whether a primary or best available time source has failed based on GNSS location. In embodiments where GNSS is the best available time source, the location derived from the GNSS signal can be used a check for failure of the GNSS time source. This method is particularly useful in embodiments where the time distribution device is at a known, fixed location. In one embodiment, the known location of the time distribution device can be entered by a user at the time of setup and can be modified as necessary. In another embodiment, the known location of the time distribution device can be calculated using GNSS signals.

At 602 the time distribution device receives the GNSS signal. While the example of FIG. 6 is described in terms of a single GNSS signal for clarity, one of ordinary skill in the art will recognize that multiple signals from various GNSS satellites are typically used in determining GNSS receiver location and can be used to more accurately calculate GNSS receiver location. At 604, the GNSS receiver calculates the location of the time distribution device based on the received GNSS signal. The time quality module, at 606, compares the calculated location of the time distribution device with the known location of the time distribution device and determines whether the calculated location falls within a threshold distance from the known location. Because GNSS location calculation varies based on the techniques employed by the GNSS receiver, the threshold distance can vary from device to device.

If, at 608, the time quality module determines that the GNSS location falls within the threshold, the time quality module distributes the GNSS time as the precision time reference at 610. If, at 608, the time quality module determines that the GNSS location falls outside of the threshold and therefore the GNSS time source has failed, at 612 the time quality module alerts a user that the best available time source has failed and that the time may not be accurate. In addition to alerting a user of the failure, the time quality module at 614 can optionally distribute time from a secondary time source as the precision time reference during a holdover period.

In another embodiment, the time quality module may calculate a location drift rate using the GNSS signal and compare the location drift rate to a defined threshold. If the location drift rate exceeds the defined threshold, the time quality module may determine, at 608, that the GNSS time source has failed.

In one embodiment, the time quality module monitors instantaneous and average GNSS signal strength. If the instantaneous signal strength is larger than a set threshold for a set number of samples, then the time quality module may determine that the GNSS time source has failed. In such an instance, the time quality module may alert a user and/or rely upon a secondary time signal.

In another embodiment, satellite constellation may be monitored. Satellite constellation repeats every 24 hours. The time quality module may determine that the GNSS time source has failed by detecting a change in satellite constellation. In such an instance, the time quality module may alert a user and/or rely on a secondary time signal.

The above description provides numerous specific details for a thorough understanding of the embodiments described herein. However, those of skill in the art will recognize that one or more of the specific details may be omitted, or other methods, components, or materials may be used. In some cases, operations are not shown or described in detail.

While specific embodiments and applications of the disclosure have been illustrated and described, it is to be understood that the disclosure is not limited to the precise configuration and components disclosed herein. Various modifications, changes, and variations apparent to those of skill in the art may be made in the arrangement, operation, and details of the methods and systems of the disclosure without departing from the spirit and scope of the disclosure. 

What is claimed is:
 1. A method comprising: receiving, at a time distribution device, a first time signal from a first precision time source via a first time receiver; distributing, by the time distribution device a time signal to a time-dependent device, wherein the time-dependent device is dependent on the time distribution device for a precision time reference; receiving, at the time distribution device, a second time signal from a second precision time source via a second time receiver, the second time signal independent from the first precision time source; comparing, by a time quality component of the time distribution device, the first time signal with the second time signal; detecting, by the time quality component of the time distribution device, a failure of the first precision time source in response to the comparing showing that a variation of the first time signal from the second time signal exceeds a defined margin; and, upon detection of the failure of the first precision time source, the time distribution device distributing to the time-dependent device the time signal based on the second time signal.
 2. the method of claim 1, wherein the first precision time source is a global navigation satellite system (GNSS) time source and the first time signal is a GNSS pulse per-second (PPS) signal.
 3. The method of claim 1, wherein the second precision time source is a WWVB time source and the second time signal is a WWVB PPS.
 4. The method of claim 1, further comprising: in response to detecting a failure of the first precision time source, relying on the second precision time source.
 5. A system comprising: a first receiver of a time distribution device, configured to receive a first signal including a first precision time signal; a second receiver of the time distribution device configured to receive a second signal including a second precision time signal independent from the first precision time signal, wherein the second precision time signal is relatively less precise than the first precision time signal; an output of the time distribution device, configured to provide an output time signal to a time-dependent device, wherein the time-dependent device is dependent on the time distribution device for a precision time reference; and a time quality module of the time distribution device, configured to compare the first precision time signal with the second precision time signal; detect an error condition of the first precision time signal in response to a variation of the first precision time signal from the second precision time signal exceeding a defined threshold; distribute to the time-dependent device a time signal based on the first precision time signal; and, upon detection of the error condition of the first precision time signal, distribute to the time-dependent device a time signal based on the second precision time signal; and, the time-dependent device in communication with the time distribution device, configured to receive the output time signal from the time distribution device.
 6. the system of claim 5, wherein the first signal is a global navigation satellite system (GNSS) signal.
 7. The system of claim 5, wherein the second signal is a WWVB signal.
 8. The system of claim 5, wherein the second signal is a network time signal.
 9. The system of claim 5 further comprising, an unlocked oscillator trained to the first precision time signal, wherein the second precision time signal comprises a time signal from the unlocked oscillator.
 10. The system of claim 5, wherein, in response to detecting an error condition, the output is configured to provide the second precision time signal to the IED.
 11. A method comprising: receiving a global navigation satellite system (GNSS) signal, including a GNSS time signal, at a time distribution device; distributing, by the time distribution device, a time signal to a time-dependent device, wherein the time-dependent device is dependent on the time distribution device for the time signal; determining, by the time distribution device, whether the GNSS has failed; in response to determining that the GNSS has failed, indicating to a user an error condition; in response to determining that the GNSS has not failed, distributing to the time-dependent device the time signal based on the GNSS time signal; and, in response to determining that the GNSS has failed, distributing to the time-dependent device a time signal not based on the GNSS.
 12. The method of claim 11, wherein determining whether the GNSS has failed comprises: comparing a location determined from the GNSS signal with a known location of the time distribution device; and determining that the GNSS has failed in response to the GNSS location varying from the known location by more than a defined threshold.
 13. The method of claim 11, wherein determining whether the GNSS has failed comprises: comparing the GNSS time signal with an independent time signal; and determining that the GNSS has failed in response to the GNSS time signal varying from the independent time signal by more than a defined threshold.
 14. The method of claim 13, wherein the independent time signal is a National Institute of Science and Technology (NIST) broadcast signal.
 15. The method of claim 13, wherein the independent time signal is generated by an oscillator.
 16. The method of claim 13, wherein the independent time signal is received via a network protocol.
 17. The method of claim 13, further comprising: in response to determining that the GNSS has failed, distributing to the time-dependent device a time signal based on the independent time signal.
 18. The method of claim 11, wherein determining whether the GNSS has failed comprises: calculating a location drift rate based on the GNSS signal; comparing the location drift rate to a defined threshold; and determining that the GNSS has failed in response to the location drift rate exceeding the defined threshold.
 19. The method of claim 11, wherein determining whether the GNSS has failed comprises: monitoring instantaneous and average GNSS signal strength; and determining that the GNSS has failed in response to the instantaneous signal strength exceeding a defined threshold for a set number of samples.
 20. The method of claim 11, wherein determining whether the GNSS has failed comprises: monitoring satellite constellation; and determining that the GNSS time source has failed in response to detecting a change in satellite constellation. 